In general, high-voltage electrical insulating materials for use as insulators and bushings for power transmission lines are of porcelain (ceramics) or glass. In a polluted environment as in seaside areas and industrial areas, there is a tendency that dust, salts and mist attach to the surface of high-voltage electrical insulators, causing leakage currents and dry band discharge leading to flashover failure.
In order to eliminate the drawbacks of porcelain and glass insulators, a number of proposals have been made. For example, U.S. Pat. No. 3,511,698 discloses a weathering resistant high-voltage electrical insulator comprising a member of a thermosetting resin and a platinum catalyst-containing organopolysiloxane elastomer. JP-A 198604/1984 corresponding to U.S. Pat. No. 4,476,155 proposes a one-part room temperature curable organopolysiloxane composition which is applied to the outer surface of an electrical insulator of glass or porcelain so that the electrical insulator may maintain its high insulating properties even in the presence of moisture, polluted air, ultraviolet radiation and other outdoor stresses.
JP-B 35982/1978 corresponding to U.S. Pat. No. 3,965,065 and JP-A 209655/1992 corresponding to U.S. Pat. No. 5,369,161 disclose that a silicone rubber composition with improved electrical insulation is obtained by heating a mixture of an organopolysiloxane capable of heat curing into silicone rubber and aluminum hydrate at temperatures above 100.degree. C. for more than 30 minutes.
However, the silicone rubber compositions mentioned above are not yet fully satisfactory in high-voltage electrical insulation under rigorous conditions. Silicone rubber compositions loaded with large amounts of aluminum hydrate have a higher moisture pickup than unloaded silicone rubber since aluminum hydrate itself is hygroscopic. Thus the loaded compositions lose electrical properties in humid or wet conditions. The moisture pickup gives rise to another problem that the corona resistance required for high-voltage electrical insulators is lost. This problem cannot be solved simply by surface treating aluminum hydrate with chemical agents. There is a desire to solve this and other problems.